1. Field of the Invention
The present invention relates to a lens meter, and more particularly to the lens meter capable of measuring optical characteristics of lenses to be examined, for instance spectacle lenses, contact lenses and the like.
2. Description of Related Art
Known is a measurement principle of lens meters such that a target provided with a predetermined pattern is disposed in front or behind of a lens to be examined and a light receiving element detects the optical image of the target deflected by passing through the lens to be examined, so that optical characteristics of the lens are obtained based on the detected results of the light receiving element.
When the position of the target is fixed in such a kind of lens meter, the optical image of the target formed on the light receiving element becomes a blurred image by the refractive power of the lens to be examined, the blur degree of the image becoming larger in proportional as the refractive power of the lens increases.
For preventing measuring errors from occurring due to blur of the image, there is also a lens meter in which the target is moved along an optic axis of measuring light according to the refractive power of the lens to be examined so that blur degree of the image formed on the light receiving element may be reduced.
However, the above mentioned lens meter in which the target is moved along the optic axis, the position of the optical image of the target being moved along with movement of the target, needs to detect an accurate position to which the target for carrying out high precise measurement. In the conventional lens meters having a system for moving a target along an optic axis of measuring light, it is difficult to detect always an accurate stop position of the target in reason of secular change and others. Accordingly, such a conventional apparatus is required providing with a detecting system for detecting the stop position of the target separately from the system for controlling movement of the target. This causes problems of complication in construction of the apparatus and the increased cost.